Random Access Preamble for Minimizing PA Backoff

ABSTRACT

An example method in a user equipment comprises generating a random access preamble signal and transmitting the random access preamble signal. This generating of the random access preamble signal comprises generating a random access preamble signal comprising two or more consecutive preamble symbol groups, each preamble symbol group comprising a cyclic prefix portion and a plurality of identical symbols occupying a single subcarrier of the random access preamble signal. The single subcarrier for at least one of the preamble symbol groups corresponds to a first subcarrier frequency and the single subcarrier for an immediately subsequent one of the preamble symbol groups corresponds to a second subcarrier frequency.

RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 16/203,057, filed 28 Nov. 2018, which was a continuation ofU.S. patent application Ser. No. 15/645,543, filed 10 Jul. 2017 andissued 1 Jan. 2029 as U.S. Pat. No. 10,172,163, which was a continuationof U.S. patent application Ser. No. 15/277,386, filed 27 Sep. 2016 andissued 1 Aug. 207 as U.S. Pat. No. 9,723,634, and claims priority to andthe benefit of U.S. provisional patent application Ser. No. 62/233,822,filed 28 Sep. 2015. The entire contents of the foregoing applicationsare incorporated by reference herein.

TECHNICAL FIELD

The present disclosure is generally related to wireless communicationsnetworks and is more particularly related to random access procedures inan Internet of Things (IoT) supporting machine-type-communication (MTC)devices.

BACKGROUND

Members of the 3^(rd) Generation Partnership Project (3GPP) have agreedto define specifications for what is being called “NB-IoT,” which refersto a “narrowband Internet of things.” These standards will supportwireless communications for low-power equipment that may rely onbatteries and that will typically send and receive only small amounts ofinformation. Example applications for wireless devices that supportNB-IoT include providing parking meters, industrials sensors, and thelike with wireless communication capabilities.

The radio interface for NB-IoT will be designed so that the technologycan readily be deployed by operators in portions of their existing LongTerm Evolution (LTE) spectrum. Thus, it is expected that certain aspectsof the NB-IoT will be defined to make the most possible use of existingLTE hardware, designs, and procedures. However, changes to the LTEspecifications are likely to be made at all levels of thespecifications, to reduce power consumption, improve coverage, andotherwise provide for improved operation of low-power wirelessequipment.

One aspect of the existing LTE specifications is random access. In LTE,as in most communication systems, a mobile terminal may need to contactthe network, via the eNodeB (3GPP terminology for an LTE base station),without yet having a dedicated resource in the uplink (from userequipment, UE, to base station). To handle this, a random accessprocedure is available, whereby a UE that does not have a dedicateduplink resource may transmit a signal to the base station. In theprocess defined by the 3GPP specifications for LTE, the first message(MSG1 or preamble) of this procedure is transmitted on a specialresource reserved for random access, a physical random access channel(PRACH). This channel is limited in time and frequency, as shown inFIG. 1. The resources available for PRACH transmissions are identifiedto mobile terminals as part of the broadcasted system information or aspart of dedicated Radio Resource Control (RRC) signaling in some cases,such as in the case of a handover.

In LTE, the random access procedure is used for a number of differentreasons. Among these reasons are:

-   -   initial access, for UEs in the LTE_IDLE or LTE_DETACHED states;    -   an incoming handover;    -   resynchronization of the uplink;    -   a scheduling request, for a UE that is not allocated any other        resource for contacting the base station; and    -   positioning.

To preserve orthogonality among different user equipments (UEs-3GPPterminology for radio access terminals, including cellular telephonesand machine-to-machine radio devices) in an orthogonalfrequency-division multiple-access (OFDMA) or single-carrierfrequency-division multiple-access (SC-FDMA) system, the time of arrivalof each UE signal needs to be within the cyclic prefix (CP) of the OFDMor SC-FDMA signal. It will be appreciated that the term cyclic prefix inbackground art refers to the prefixing of an OFDM symbol with arepetition of the symbol's end. The cyclic prefix acts as a guardinterval, so as to eliminate inter-symbol interference from the previoussymbol. It also allows the linear convolution of a channel to bemodelled as circular convolution, which can be performed in thefrequency domain with a discrete Fourier transform. Thisfrequency-domain processing simplifies demodulation processes in an LTEreceiver.

LTE random access can be either contention-based or contention-free. Thecontention-based random access procedure consists of four steps, asillustrated in FIG. 2. Note that only the first step involvesphysical-layer processing specifically designed for random access, whilethe remaining three steps follow the same physical-layer processing usedin uplink and downlink data transmission. The eNodeB can order the UE,through a Physical Downlink Control Channel (PDCCH), to perform acontention based random access. The UE starts the random accessprocedure by randomly selecting one of the preambles available forcontention-based random access. The UE then transmits the selectedrandom access preamble on the PRACH to the eNodeB in the Radio AccessNetwork (RAN), shown in FIG. 2 as step 1.

The RAN acknowledges any preamble it detects by transmitting a randomaccess response, which includes an initial grant to be used on theuplink shared channel, a temporary Cell Radio Network TemporaryIdentification (C-RNTI) for the UE, and a time alignment (TA) update.The TA update is based on the timing offset of the preamble measured bythe eNodeB on the PRACH. The random access response is transmitted inthe downlink to the UE (step 2) and its corresponding PDCCH messagecyclic redundancy code (CRC) is scrambled with a Random Access RadioNetwork Temporary Identifier (RA-RNTI).

After receiving the random access response, the UE uses the grant totransmit a message back to the RAN (step 3). This message is used, inpart, to trigger the establishment of RRC and in part to uniquelyidentify the UE on the common channels of the cell. The timing advancecommand that was provided to the UE in the random access response isapplied in the UL transmission in message transmitted back to the RAN.The eNodeB can change the resources blocks that are assigned fortransmission of this message of step 3 by sending a UL grant having itsCRC scrambled with a Temporary Cell Radio Network Temporary Identifier(TC-RNTI).

The procedure ends with the RAN solving any preamble contention that mayhave occurred for the case that multiple UEs transmitted the samepreamble at the same time. This can occur when each UE randomly selectswhen to transmit and which preamble to use. If multiple UEs select thesame preamble for the transmission at the same time on the Random AccessChannel (RACH), there will be contention between these UEs. The RANresolves this contention using the contention resolution message, seenas step 4 in FIG. 2. This message, which is sent by the eNodeB forcontention resolution, has its PDCCH CRC scrambled with the C-RNTI ifthe UE previously has a C-RNTI assigned. If the UE does not have aC-RNTI previously assigned has its PDCCH CRC is scrambled with theTC-RNTI.

A scenario where contention occurs is illustrated in FIG. 3, where twoUEs transmit the same preamble, p₅, at the same time. A third UE alsotransmits a random access preamble at the same time, but since ittransmits with a different preamble, p₁, there is no contention betweenthis UE and the other two UEs.

For contention-free random access, the UE uses reserved preamblesassigned by the base station. In this case, contention resolution is notneeded, and thus only steps 1 and 2 of FIG. 2 are required. Anon-contention-based random access or contention-free random access canbe initiated by the eNodeB, for example, to get the UE to achievesynchronization in the uplink. The eNodeB initiates anon-contention-based random access either by sending a PDCCH order orindicating it in an RRC message. The latter of these two approaches isused in the case of a handover.

The procedure for the UE to perform contention-free random access isillustrated in FIG. 4. As with the contention-based random access, therandom access response is transmitted in the downlink to the UE and itscorresponding PDCCH message CRC is scrambled with the RA-RNTI. The UEconsiders the contention resolution successfully completed after it hasreceived the random access response successfully. For thecontention-free random access, as for the contention-based randomaccess, the random access response contains a timing alignment value.This enables the eNodeB to set the initial/updated timing according tothe UEs transmitted preamble.

Efforts currently underway with respect to the so-called NetworkedSociety and Internet of Things (IoT) are associated with newrequirements on cellular networks, e.g., with respect to device cost,battery lifetime and coverage. To drive down device and module cost forthe small wireless devices that are expected to become ubiquitous, usinga system-on-a-chip (SoC) solution with integrated power amplifier (PA)is highly desirable. However, it is currently feasible forstate-of-the-art PA technology to allow only about 20-23 dBm transmitpower when the power amplified is integrated to the SoC. This constrainton output power from the SoC solution limits uplink coverage, which isrelated to how much the path loss is allowed between the user terminaland base station.

Further, to maximize the coverage achievable by an integrated PA, it isnecessary to reduce PA backoff. PA backoff is needed when thecommunication signal has a non-unity peak-to-average power ratio (PAPR),i.e., when the communication signal is not a constant envelope signal.To avoid spurious signals and out-of-band emissions from the PA whenamplifying a non-constant-envelope signal, the PA must be operated at ornear its linear operating region, i.e., it must be “backed off” from itshigh-efficiency, nonlinear operating region. The higher the PAPR is, thehigher the PA backoff required. Because higher PA backoff gives rise tolower PA efficiency, it lowers device battery life time. Thus, forwireless IoT technologies, designing an uplink communication signal thathas as low PAPR as possible is critically important for achieving theperformance objectives for IoT devices with respect to device cost,battery lifetime and coverage.

SUMMARY

Currently 3GPP is standardizing Narrow-band IoT (NB-IoT) technologies.There is a strong support from the existing LTE eco-system (vendors andoperators) for evolving existing LTE specifications to include thedesired NB-IoT features. This is motivated by the time-to-marketconsideration, since an LTE based NB-IoT solution can be standardizedand developed in a shorter time frame. A leading candidate for NB-IoT isa LTE-based NB-LTE solution.

The LTE uplink (mobile-station-to-base-station transmissions) is basedon single-carrier frequency-division multiple-access (SC-FDMA)modulation for the uplink data and control channels. For random accesspreamble transmission, a Zadoff-Chu signal is used. Neither of thesesignals has good PAPR properties.

To resolve this problem, a new random access preamble signal isdisclosed herein. This signal is appropriate for the physical randomaccess channel (PRACH) of NB-IoT. The new PRACH signal achieves 0 dBPAPR, and thus eliminates the need for PA backoff and maximizes PAefficiency. The new PRACH signal is compatible with the use of SC-FDMAand/or orthogonal frequency-division multiple-access (OFDMA) fortransmissions of uplink data and control channel signals, since the newPRACH signal, in any given OFDM symbol interval, looks like an OFDMsignal occupying only a single subcarrier. Note that for a singlesubcarrier signal, an OFDM signal is identical to the correspondingSC-FDMA signal.

Since the new PRACH signal achieves 0 dB PAPR, it eliminates the needfor PA backoff and maximizes PA efficiency. Thus, it maximizes the PRACHcoverage and battery efficiency. The new PRACH signal is compatible withSC-FDMA and orthogonal frequency-division multiple-access (OFDMA). Thus,it can be easily implemented using existing SC-FDMA or OFDMA signalgenerator. This reduces both development cost and time-to-market.

According to some embodiments, a method in a user equipment includesgenerating a Single-Carrier Frequency-Division Multiple Access (SC-FDMA)random access preamble signal comprising two or more consecutivepreamble symbol groups, each preamble symbol group comprising a cyclicprefix portion and a plurality of identical symbols occupying a singlesubcarrier of the SC-FDMA random access preamble signal. The generatingof the SC-FDMA random access preamble signal is done such that thesingle subcarrier for at least one of the preamble symbol groupscorresponds to a first subcarrier frequency and the single subcarrierfor an immediately subsequent one of the preamble symbol groupscorresponds to a second subcarrier frequency. The method furthercomprises transmitting the SC-FDMA random access preamble signal. Insome cases, the method may include selecting a preamble configurationfrom a plurality of pre-determined preamble configurations, wherein theselected preamble configuration defines the first and second subcarrierfrequencies.

According to some embodiments, a method in a base station includesreceiving a radio-frequency signal and detecting, in the radio-frequencysignal, an SC-FDMA random access preamble signal, transmitted by a firstremote user equipment, where the first SC-FDMA random access preamblesignal comprises two or more consecutive preamble symbol groups. Eachpreamble symbol group comprises a cyclic prefix portion and a pluralityof identical symbols occupying a single subcarrier of the SC-FDMA randomaccess preamble signal, such that the single subcarrier for at least oneof the preamble symbol groups corresponds to a first subcarrierfrequency and the single subcarrier for an immediately subsequent one ofthe preamble symbol groups corresponds to a second subcarrier frequency.In some cases, the method includes estimating a time-of-arrival for thefirst preamble signal.

According to the embodiments summarized, a single subcarrier signal isused in any OFDM symbol interval of the random access preamble signal.In different OFDM symbol intervals, different subcarrier (frequencies)may be used. This can be thought of as frequency hopping. This can beused ensure phase continuity between transitions (thus there isrelationship between CP duration, nominal data symbol duration, andhopping distance in frequency). In other embodiments, orthogonalfrequency-hopping patterns are designed between different PRACHpreambles, so that the random access preamble signals from differentdevices are orthogonally multiplexed and separately detectable by thereceiving base station.

According to some embodiments, a user equipment includes a radiotransceiver adapted to communicate with another user equipment and oneor more processing circuits adapted to carry out the methods in the userequipment described above. Likewise, an example base station comprises aradio transceiver adapted to communicate with one or more remote userequipments, and one or more processing circuits adapted to carry out themethods in the base station described.

Further embodiments may include computer program products andnon-transitory computer readable media that store instructions that,when executed by processing circuit, perform the operations of theembodiments describe above.

Details of several embodiments of techniques and apparatuses forperforming random access procedures are described and illustrated below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating random access preamble transmission.

FIG. 2 is a diagram illustrating signaling for the contention-basedrandom access procedure in LTE.

FIG. 3 illustrates contention based random access, where there iscontention between UEs.

FIG. 4 is a diagram illustrating signaling over the air interface forthe contention-free random access procedure in LTE.

FIG. 5 is a diagram illustrating a cyclic prefix, guard period andpreamble sequence for PRACH.

FIG. 6 illustrates an example PRACH signal for one OFDM symbol.

FIG. 7 is a diagram illustrating an example PRACH signal over multipleOFDM symbol intervals, according to some embodiments.

FIG. 8 is a diagram illustrating frequency multiplexing of two PRACHpreambles, according to some embodiments.

FIG. 9 illustrates frequency multiplexing of two PRACH preambles(time-domain signal), according to some embodiments.

FIG. 10 is a diagram illustrating an example of CP duration that is onefourth of the OFDM data symbol duration, according to some embodiments.

FIG. 11 is a block diagram of a user equipment configured to perform arandom access procedure, according to some embodiments.

FIG. 12 is a flowchart illustrating a method in a user equipment forperforming a random access procedure, according to some embodiments.

FIG. 13 is a block diagram of a network node configured to signalinformation pertaining to a random access procedure, according to someembodiments.

FIG. 14 is a flowchart illustrating a random access procedure, accordingto some embodiments.

FIG. 15 is a block diagram of a functional implementation of a userequipment for performing a random access procedure, according to someembodiments.

FIG. 16 is a block diagram of a functional implementation of a networknode for receiving reports pertaining to a random access procedure,according to some embodiments.

DETAILED DESCRIPTION

Inventive concepts will now be described more fully hereinafter withreference to the accompanying drawings, in which examples of embodimentsof inventive concepts are shown. These inventive concepts may, however,be embodied in many different forms and should not be construed aslimited to the embodiments set forth herein. Rather, these embodimentsare provided so that this disclosure will be thorough and complete, andfully convey the scope of present inventive concepts to those skilled inthe art. It should also be noted that these embodiments are not mutuallyexclusive. Components from one embodiment may be tacitly assumed to bepresent or used in another embodiment.

For purposes of illustration and explanation only, embodiments of thepresent inventive concepts are described herein in the context ofoperating in or in association with a RAN that communicates over radiocommunication channels with mobile terminals, also interchangeablyreferred to as wireless terminals or UEs, using a particular radioaccess technology. More specifically, embodiments are described in thecontext of the development of specifications for NB-IoT, particularly asit relates to the development of specifications for NB-IoT operation inspectrum and/or using equipment currently used by E-UTRAN, sometimesreferred to as the Evolved UMTS Terrestrial Radio Access Network andwidely known as the LTE system. However, it will be appreciated that thetechniques may be applied to other wireless networks, as well as tosuccessors of the E-UTRAN. Thus, references herein to signals usingterminology from the 3GPP standards for LTE should be understood toapply more generally to signals having similar characteristics and/orpurposes, in other networks.

Note that in some of the embodiments described herein, the terms “userequipment” and “UE” are used. A UE, as that term is used herein, can beany type of wireless device capable of communicating with a network nodeor another UE over radio signals. In the context of the presentdisclosure, it should be understood that a UE may refer to amachine-to-machine (M2M) device, a machine-type communications (MTC)device, and/or a NB-IoT device, where the UE has no “user” in the senseof an individual person owning and/or operating the device. A UE mayalso be referred to as a wireless device, a radio device, a radiocommunication device, a wireless terminal, or simply a terminal—unlessthe context indicates otherwise, the use of any of these terms isintended to include device-to-device UEs, machine-type UEs or UEscapable of machine-to-machine communication, sensors equipped with a UE,wireless-enabled table computers, mobile terminals, smart phones,laptop-embedded equipped (LEE), laptop-mounted equipment (LME), USBdongles, wireless customer-premises equipment (CPE), etc. In thediscussion that follows, the terms machine-to-machine (M2M) device,machine-type communication (MTC) device, wireless sensor, and sensor mayalso be used. It should be understood that these devices are UEs, butare generally configured to transmit and/or receive data without directhuman interaction.

In the existing LTE random access design, random access serves multiplepurposes such as initial access when establishing a radio link,scheduling request, etc. Among others, a main objective of random accessis to achieve uplink synchronization, which is important for maintainingthe uplink orthogonality in LTE. To preserve orthogonality amongdifferent user equipments (UEs) in an OFDM or SC-FDMA system, the timeof arrival of each UE signal needs to be within the cyclic prefix (CP)of the OFDM or SC-FDMA signal.

As discussed in the Background section above, a cyclic prefix (CP) isused to provide a guard time between consecutive symbols and, especiallyin the context of OFDMA and/or SC-FDMA transmissions, to simplify thereceiver processing. FIG. 5 illustrates how a CP could be used for atransmitted OFDMA/SC-FDMA symbol that forms all or part of physicalrandom access channel (PRACH) preamble sequence. As shown in FIG. 5, aPRACH preamble sequence is sent by the UE during a random access timesegment illustrated in FIG. 5. In the illustrated example, thetransmission has a duration of 3.6 milliseconds, including a CP durationof 400 microseconds and a data interval of 3.2 milliseconds. The PRACHpreamble sequence does not occupy the entire random access segment,leaving some time as guard time and also allow a cyclic prefix (CP)interval.

As discussed above, 3GPP is defining specifications for NB-IoT, whichwill support wireless communications for low-power equipment that mayrely on batteries and that will typically send and receive only smallamounts of information. It is desirable that the specifications forNB-IoT, where possible, facilitate the re-use of existing designs andtechniques, and facilitate deployment in existing LTE spectrum. Thepreviously existing LTE uplink (mobile-station-to-base-stationtransmissions), however, is based on single-carrier frequency-divisionmultiple-access (SC-FDMA) modulation for the uplink data and controlchannels. For random access preamble transmission, a Zadoff-Chu signalis used. Neither of these signals has good PAPR properties, however,which creates problems for low-power ad low-cost devices, especiallythose relying on an integrated system-on-chip (SoC) system.

To resolve this problem, a new random access preamble signal isdisclosed herein. This signal is appropriate for the physical randomaccess channel (PRACH) of NB-IoT. The new PRACH signal achieves 0 dBPAPR, and thus eliminates the need for PA back-off and maximizes PAefficiency. The new PRACH signal is compatible with the use of SC-FDMAand/or orthogonal frequency-division multiple-access (OFDMA) fortransmissions of uplink data and control channel signals, since the newPRACH signal, in any given OFDM symbol interval, looks like an OFDMsignal occupying only a single subcarrier. Note that for a singlesubcarrier signal, an OFDM signal is identical to the correspondingSC-FDMA signal.

Since the new PRACH signal achieves 0 dB PAPR, it eliminates the needfor PA back-off and maximizes PA efficiency. Thus, it maximizes thePRACH coverage and battery efficiency. The new PRACH signal iscompatible with SC-FDMA and orthogonal frequency-divisionmultiple-access (OFDMA). Thus, it can be easily implemented usingexisting SC-FDMA or OFDMA signal generator. This reduces bothdevelopment cost and time-to-market.

In FIG. 5, two PRACH preamble transmissions are shown, with one comingfrom a UE close to the eNB (LTE terminology for a node that includesradio base station functionality) and the other from a UE far from thebase station, at the cell edge. It can be seen that this results in adifference in timing for the two transmissions, relative to a randomaccess interval maintained in the eNB receiver.

The use of the CP allows the receiver to perform a circular convolutionusing, in this example, a 3.2-millisecond portion of the signal,centered in a 4-millisecond random access interval window. The eNBreceiver will have similar performance for both the near-eNB andnear-cell-edge cases.

As discussed earlier, to maximize PA efficiency and coverage, it isdesirable to have PRACH preambles as close to constant-envelope aspossible. A constant-envelope signal has 0 dB PAPR, and does not requirePA back-off. In the below description, we will use PRACH signal andPRACH preamble interchangeably.

LTE random access can be either contention-based or contention-free. Thecontention-based random access procedure consists of four steps, asillustrated in FIG. 2 and discussed above. Note that only the first stepinvolves physical-layer processing specifically designed for randomaccess, while the remaining three steps follow the same physical-layerprocessing used in uplink and downlink data transmission. Forcontention-free random access, the UE uses reserved preambles assignedby the base station. In this case, contention resolution is not needed,and thus only steps 1 and 2 are required. The techniques for randomaccess preamble transmission discussed below may be used in either orboth contention-free and contention-based random access procedures.

An example PRACH signal during a single OFDM symbol interval, accordingto some embodiments of the presently disclosed techniques, is shown inFIG. 6. It is basically a single-tone (single-subcarrier) OFDM signal.According to the example in FIG. 6, the subcarrier spacing is 2.5 kHz.However, the techniques described herein may be applied to anysubcarrier spacing.

According to some embodiments of the presently disclosed techniques, thePRACH signal is spread in time over multiple OFDM symbols, instead ofspread in frequency (as in the LTE case). Thus, a number of OFDMsymbols, each one as illustrated in FIG. 6, are concatenated to form aPRACH preamble. As will be discussed in further detail below, in someembodiments the generated random access preamble signal comprises two ormore, or N, consecutive preamble symbols, also referred to herein aspreamble symbol groups or symbol groups, with each preamble symbol groupcomprising a plurality of duplicated OFDM symbols and being formed toproduce a single tone in the transmitted random access preamble signal.In other words, each preamble symbol group comprises a plurality ofidentical symbols occupying a single subcarrier of the SC-FDMA randomaccess preamble signal, such that the single subcarrier for at least oneof the preamble symbol groups corresponds to a first subcarrierfrequency and the single subcarrier for an immediately subsequent one ofthe preamble symbol groups corresponds to a second subcarrier frequency.The subcarrier frequency changes between preamble symbol groups, suchthat the single tone for a first one of the consecutive symbol groupscorresponds to a first subcarrier frequency and the single tone for asubsequent one of the symbol groups corresponds to a second subcarrierfrequency. In other words, each preamble symbol group comprises aplurality of identical symbols occupying a single subcarrier of theSC-FDMA random access preamble signal, such that the single subcarrierfor at least one of the preamble symbol groups corresponds to a firstsubcarrier frequency and the single subcarrier for an immediatelysubsequent one of the preamble symbol groups corresponds to a secondsubcarrier frequency.

In some embodiments, the subcarrier frequencies change according to asimple pattern, where the single tone for every second preamble symbolcorresponds to a first subcarrier frequency and the single tone for theremaining preamble symbols corresponds to a second subcarrier frequency.Thus, in these embodiments, the preamble signal hops between twosubcarrier frequencies, from one preamble symbol group to the next. Itwill be appreciated, of course that other patterns are possible.

As explained in further detail below, each of the consecutive preamblesymbols may be formed by repeating a basic OFDM symbol a plurality oftimes. It should be understood that the term symbol group, as usedherein, may refer to a preamble symbol group formed in such a manner;thus a preamble symbol does not correspond to a basic OFDM symbol, butinstead may comprise a plurality of duplicated OFDM symbols. As notedabove, a single-tone OFDM signal is also a SC-FDMA signal, so theseduplicated OFDM symbols may also be understood to be SC-FDMA symbols.

An example random access preamble signal design is shown in FIG. 7. Inthis example, the PRACH preamble consists of 100 preamble symbols intime and occupies one tone/subcarrier (of 2.5 kHz) in frequency, for anygiven preamble symbol interval. However, the transmission in thisexample hops between two adjacent tones from one preamble symbol groupto the next. This hopping is used to enable satisfactory time-of-arrivalestimation performance at the base station. As noted above, this 2.5 kHzis simply an example—other subcarrier spacings are possible. Further, itshould be apparent that the tones need not be adjacent—the hopping canskip several subcarriers.

Since the tone (subcarrier) bandwidth/spacing in this example is 2.5kHz, the duration of the data part of a normal OFDM symbol would be 400microseconds, according to the well-known relationship betweensubcarrier spacing and OFDM symbol length. To support a cell size of upto, for example, 60 km, a CP of length 400 microseconds is needed toaccommodate the maximum round-trip delay. A direct transmission of a 400microsecond data part and a 400 microsecond CP would lead to a 50% CPoverhead out of the total resource. To reduce the overhead, a basic OFDMsymbol is repeated four times, in the example illustrated in FIG. 7,resulting in a 1600-microsecond symbol duration. The first copy of theOFDM symbol is treated by the base station receiver as CP, while theremaining three copies are treated as data. This design reduces the CPoverhead from 50% down to 25%. The base station can coherently combinethe three copies of the symbol and thereby obtain about 4.8 dB powergain.

To see the 0 dB PAPR property of the preamble in FIG. 7, consider,without loss of generality, OFDM symbols 1 and 2 that can be written as:

${{x(t)} = {{x\lbrack 1\rbrack}e^{j\; 2\; \pi \; \frac{k}{T}t}}},{t \in \left\lbrack {0,{4T}} \right\rbrack}$${{x(t)} = {{x\lbrack 2\rbrack}e^{j\; 2\; \pi \frac{k + 1}{T}t}}},{t \in \left\lbrack {{4T},\ {8T}} \right\rbrack}$

where T=400 microseconds and k is the subcarrier index, for subcarrierswith subcarrier spacings of 1/T. Within each OFDM symbol of length 4T,the waveform is of constant envelope, since within time intervals [0,4T] and [4T, 8T] the signal is sinusoidal. At symbol boundary, the phasedifference is

${{phase}\mspace{14mu} \left( {{x^{*}\lbrack 1\rbrack}e^{{- j}\; 2\; \pi \; \frac{k}{T}4T}{x\lbrack 2\rbrack}e^{i^{2\frac{k + 1}{T}4T}}} \right)} = {{phase}\mspace{14mu} \left( {{x^{*}\lbrack 1\rbrack}{x\lbrack 2\rbrack}} \right)}$

Therefore, sending a constant sequence, i.e., where x[1]=x[2], thatalternates between the two tones, guarantees phase continuity and yields0 dB PAPR theoretically.

Since each PRACH preamble effectively only uses one 2.5 kHz subcarrierat any given time, different preambles can be multiplexed in thefrequency domain. For example, FIG. 8 shows the multiplexing of twoPRACH preambles. In general, M tones can be configured for multiplexingM PRACH preambles. Each PRACH preamble uses one tone during one OFDMsymbol interval, and the multiplexing pattern (e.g., as shown in FIG. 8)ensures that no two UEs use the same tone during the same OFDM symbolinterval.

FIG. 8 illustrates the frequency-domain arrangement of the two PRACHpreambles. Examples of the corresponding time-domain signals are shownin FIG. 9, where an interval of about four preamble symbol groups isshown for each preamble signal. First, it can be seen thatphase-continuity is preserved in each of the preambles. Second, it canbe seen that preamble 1 starts with a lower-frequency sinusoidal,switching to a higher-frequency sinusoidal, the lower-frequencysinusoidal, and finally switching again to a higher-frequencysinusoidal. Preamble 2 starts with the higher-frequency sinusoidal,switching to the lower-frequency sinusoidal, higher-frequencysinusoidal, and finally switching again to the lower-frequencysinusoidal. These two preambles are orthogonal to each other if theirdifferential arrival time at the base station is within the CP interval.Note that in both of the illustrated examples, there is phase continuitybetween the lower-frequency and higher-frequency sinusoids. It will beappreciated that the low-to-high and high-to-low sequences of the twopreambles may be pre-configured in each of the radio devices sending thepreambles, or may result from a random selection of a configuration bythe radio devices.

It will be appreciated that the presently disclosed techniques can begeneralized to any CP duration, or any relationship between the CPduration and normal data duration within a preamble symbol group.However, the hopping distance in frequency should be adjustedaccordingly, to maintain phase continuity at OFDM symbol boundarieswhere transitioning between frequency tones occurs. This is important inmaintaining the constant envelope property.

An example is given in FIG. 10. Here the subcarrier spacing (tonebandwidth) is still 2.5 kHz, and thus the nominal data symbol duration(T) is 1/2500, i.e. 400 microseconds, which, in an OFDM context, impliesa subcarrier spacing of 2.5 kHz. As shown, the CP is 100 us, and thus isone fourth of the nominal data symbol duration. Note that the OFDMsymbol duration is thus 1.25T (CP plus data). Consider OFDM symbols 1and 2, which can be written as

${{{x(t)} = {{x\lbrack 1\rbrack}e^{j\; 2\; \pi \; \frac{k}{T}t}}},{t \in \left\lbrack {0,{{1.2}5T}} \right\rbrack}}{{{x(t)} = {{x\lbrack 2\rbrack}e^{i^{2\frac{k + 4}{T}t}}}},{t \in \left\lbrack {{{1.2}5T},\ {{2.5}T}} \right\rbrack}}$

where T=400 us. Within each OFDM symbol of length 1.25T, the waveform isof constant envelope. At symbol boundary, the phase difference is

${{phase}\mspace{14mu} \left( {{x^{*}\lbrack 1\rbrack}e^{{- j}\; 2\; \pi \; \frac{k}{T}{({1.25\; T})}}{x\lbrack 2\rbrack}e^{j\; 2\; \pi \; \frac{k + 4}{T}{({1.25\; T})}}} \right)} = {{phase}\mspace{14mu} \left( {{x^{*}\lbrack 1\rbrack}{x\lbrack 2\rbrack}} \right)}$

Therefore, sending a constant sequence, i.e., where x[1]=x[2], thatalternates between two subcarriers that are 4 tones apart, guaranteesphase continuity and yields 0 dB PAPR theoretically.

FIG. 11 shows an example radio device, here illustrated as a UE 12,which may be more generally referred to a wireless terminal and whichcan be used in one or more of the example embodiments described herein.The UE 12 may in some embodiments be a mobile device that is configuredfor operation according to specifications for NB-IoT. The UE 12comprises a processing circuit 30 that controls the operation of the UE12. The processing circuit 30, which may comprise one or moremicroprocessors, microcontrollers, digital signal processors,specialized digital logic, etc., for example, is connected to a receiveror transceiver circuit 32 with associated antenna(s) 34, which are usedto receive signals from or both transmit signals to and receive signalsfrom a base station 10 in the network 2. The UE 12 also comprises amemory circuit 36 that is connected to the processing circuit 30 andthat stores program code and other information and data required for theoperation of the UE 12. Together, the processing circuit 30 and memorycircuit 36 may also be referred to as a processing circuit, and areadapted, in various embodiments, to carry out one or more of theUE-based techniques described herein.

For example, the processing circuit of UE 12 may be configured togenerate a SC-FDMA random access preamble signal comprising two or moreconsecutive preamble symbol groups, each preamble symbol groupcomprising a cyclic prefix portion and a plurality of identical symbolsoccupying a single subcarrier of the SC-FDMA random access preamblesignal, such that the single subcarrier for at least one of the preamblesymbol groups corresponds to a first subcarrier frequency and the singlesubcarrier for an immediately subsequent one of the preamble symbolgroups corresponds to a second subcarrier frequency. The processingcircuit of UE 12 is further configured to transmit the random accesspreamble signal. As discussed in the examples described above, thesingle subcarrier for at least one of the preamble symbol groupscorresponds to a first subcarrier frequency and the single subcarrierfor an immediately subsequent one of the preamble symbol groupscorresponds to a second subcarrier frequency. In some embodiments, everysecond one of the preamble symbol groups corresponds to the secondsubcarrier frequency and each of the remaining preamble symbol groupscorresponds to the first subcarrier frequency.

Regardless of the implementation, the processing circuit of UE 12 isconfigured to perform a method 1200 as shown in FIG. 12. As shown atblock 1220, the method 1200 includes generating an SC-FDMA random accesspreamble signal comprising two or more consecutive preamble symbolgroups, each preamble symbol group comprising a cyclic prefix portionand a plurality of identical symbols occupying a single subcarrier ofthe SC-FDMA random access preamble signal, such that the singlesubcarrier for at least one of the preamble symbol groups corresponds toa first subcarrier frequency and the single subcarrier for animmediately subsequent one of the preamble symbol groups corresponds toa second subcarrier frequency; in some embodiments, every second one ofthe preamble symbol groups corresponds to the second subcarrierfrequency and each of the remaining preamble symbol groups correspondsto the first subcarrier frequency. The method 1200 also includestransmitting the random access preamble signal, as shown at block 1230.In some cases, the method 1200 may include selecting a preambleconfiguration from a plurality of pre-determined preambleconfigurations, where the selected preamble configuration defines thefirst and second subcarrier frequencies, as shown at block 1210, andwhere the SC-FDMA random access preamble signal is generated using theselected preamble configuration. This selection may be performedrandomly, in some embodiments.

In some embodiments, all of the preamble symbol groups have the samecomplex amplitude. In some embodiments, the first and second subcarrierfrequencies are selected so as to enable phase continuity at theboundaries between preamble symbols. The preamble symbol groups are thengenerated so as to provide phase continuity at the boundaries betweenpreamble symbol groups.

In some embodiments, the second subcarrier frequency is adjacent to thefirst subcarrier frequency. In some of these and in other embodiments,the length of the cyclic prefix portion is the same as the length ofeach of the identical symbols, and the cyclic prefix portion isidentical to each of the identical symbols. In others, the length of thecyclic prefix portion is one quarter of the length of each of theidentical symbols.

In some embodiments, each preamble symbol group has a total length of1600 microseconds. In some embodiments, the plurality of identicalsymbols in each preamble symbol group consists of three identicalsymbols.

FIG. 13 shows another example radio device, in this case illustrating anetwork node, such as a base station 10, that is configured to receive arandom access preamble signal from the UE 12. In the description of someembodiments below, the terminology “radio network node” or simply“network node” or “NW node” is used. These terms refer to any kind ofnetwork node in the fixed portion of the wireless communication network,such as a base station, a radio base station, a base transceiverstation, a base station controller, a network controller, an evolvedNode B (eNodeB or eNB), a Node B, a relay node, a positioning node, aE-SMLC, a location server, a repeater, an access point, a radio accesspoint, a Remote Radio Unit (RRU) Remote Radio Head (RRH), amulti-standard radio (MSR) radio node such as MSR base station nodes indistributed antenna system (DAS), a SON node, an O&M, OSS, or MDT node,a core network node, an MME, etc. As can be seen from these example, theterm “fixed portion” of the wireless communication network is meant torefer to the portion of the wireless network other than the accessterminals, i.e., the portion of the network that is accessed through aradio link by UEs, NT-IoB devices, and the like, and is not meant topreclude the possibility that one or more elements in a given scenariocan be moved.

FIG. 13 shows a base station 10 (for example an eNB) that can be used insome of the example embodiments described herein. It will be appreciatedthat although a macro eNB will not, in practice, be identical in sizeand structure to a micro eNB, for the purposes of illustration, the basestations 10 are assumed to include similar components. Thus, whether ornot base station 10 corresponds to a macro base station or a micro basestation, it comprises a processing circuit 40 that controls theoperation of the base station 10. The processing circuit 40, which mayinclude one or more microprocessors, microcontrollers, digital signalprocessors, specialized digital logic, etc., is connected to atransceiver circuit 42 with associated antenna(s) 44 that are used totransmit signals to, and receive signals from, UEs 12 in the network.The base station 10 also comprises a memory circuit 46 that is connectedto the processing circuit 40 and that stores program and otherinformation and data required for the operation of the base station 10.Together, the processing circuit 40 and memory circuit 46 may also bereferred to as a processing circuit, and are adapted, in variousembodiments, to carry out one or more of the network-based techniquesdescribed below.

Base station 10 also includes components and/or circuitry 48 forallowing the base station 10 to exchange information with other basestations 10 (for example, via an X2 interface) and components and/orcircuitry 49 for allowing the base station 10 to exchange informationwith nodes in the core network (for example, via an Si interface). Itwill be appreciated that base stations for use in other types of network(e.g., UTRAN or Wideband Code Division Multiple Access or WCDMA RAN)will include similar components to those shown in FIG. 13 andappropriate interface circuitry 48, 49 for enabling communications withthe other network nodes in those types of networks (e.g., other basestations, mobility management nodes and/or nodes in the core network).

The processing circuit of base station 10 is configured to receive aradio-frequency signal and detect, in the radio-frequency signal, afirst SC-FDMA random access preamble signal, transmitted by a firstremote radio device. The first SC-FDMA random access preamble signalcomprises two or more consecutive preamble symbols (which may also bereferred to as preamble symbol groups), each preamble symbol groupcomprising a cyclic prefix portion and a plurality of identical symbolsoccupying a single subcarrier of the SC-FDMA random access preamblesignal, such that the single subcarrier for at least one of the preamblesymbol groups corresponds to a first subcarrier frequency and the singlesubcarrier for an immediately subsequent one of the preamble symbolgroups corresponds to a second subcarrier frequency. In someembodiments, every second one of the preamble symbol groups correspondsto the second subcarrier frequency and each of the remaining preamblesymbol groups corresponds to the first subcarrier frequency. In somecases, the processing circuit is configured to estimate atime-of-arrival for the first preamble signal.

Regardless of the implementation, the processing circuit of base station10 is also configured to perform a method 1400, as shown in FIG. 14. Themethod 1400 includes receiving a radio-frequency signal (Block 1410).The method 1400 also includes detecting, in the radio-frequency signal,a first SC-FDMA random access preamble signal, transmitted by a firstremote radio device (Block 1420). The first random access preamblesignal comprises two or more consecutive preamble symbol groups, eachpreamble symbol group comprising a cyclic prefix portion and a pluralityof identical symbols occupying a single subcarrier of the SC-FDMA randomaccess preamble signal, such that the single subcarrier for at least oneof the preamble symbol groups corresponds to a first subcarrierfrequency and the single subcarrier for an immediately subsequent one ofthe preamble symbol groups corresponds to a second subcarrier frequency.Again, in some embodiments, every second one of the preamble symbolgroups corresponds to the second subcarrier frequency and each of theremaining preamble symbol groups corresponds to the first subcarrierfrequency. Optionally, the method 1400 includes estimating atime-of-arrival for the first SC-FDMA random access preamble signal(Block 1430)—this may be used for performing uplink synchronization, forexample.

In some embodiments, all of the preamble symbol groups have the samecomplex amplitude. Further, the first and second subcarrier frequenciesmay be selected so as to enable phase continuity at the boundariesbetween preamble symbol groups, where the detected preamble symbols havephase continuity at the boundaries between preamble symbol groups. Insome embodiments, the second subcarrier frequency is adjacent to thefirst subcarrier frequency.

In some embodiments, the length of the cyclic prefix portion is the sameas the length of each of the identical symbols and the cyclic prefixportion is identical to each of the identical symbols. In otherembodiments, the length of the cyclic prefix is one quarter of thelength of each of the identical symbols. In some embodiments, eachpreamble symbol group has a total length of 1600 microseconds; theplurality of identical symbols in each preamble symbol group consists ofthree identical symbols, in some embodiments.

As discussed above, different random access preamble signals may beinterleaved in the frequency domain, such that they may be distinguishedfrom one another by the base station. Accordingly, some embodiments ofthe method 1400 may further comprise detecting, in the radio-frequencysignal, a second SC-FDMA random access preamble signal, transmitted by asecond remote user equipment, where the second SC-FDMA random accesspreamble signal comprises two or more consecutive preamble symbolgroups, each preamble symbol group comprising a cyclic prefix portionand a plurality of identical symbols occupying a single subcarrier ofthe SC-FDMA random access preamble signal, such that the singlesubcarrier for at least one of the preamble symbol groups corresponds toa third subcarrier frequency and the single subcarrier for animmediately subsequent one of the preamble symbol groups corresponds toa fourth subcarrier frequency. In these embodiments, the two or moreconsecutive preamble symbol groups of the second SC-FDMA random accesspreamble signal may overlap, at least partly, the two or moreconsecutive preamble symbol groups of the first SC-FDMA random accesspreamble, and the first subcarrier frequency may equal the fourthsubcarrier frequency, or the second subcarrier frequency may equal thethird subcarrier frequency, or both.

It should be understood that the methods 1200 and 1400 illustrated inFIGS. 12 and 14 are examples of the techniques described more fullyabove. Each of these methods may be modified according to any of thevariations and details discussed. The methods illustrated in FIGS. 12and 14, and variants thereof, may be implemented using the processingcircuits illustrated in FIGS. 11 and 13, as appropriate, where theprocessing circuits are configured, e.g., with appropriate program codestored in memory circuits 36, and/or 46, to carry out the operationsdescribed above. While some of these embodiments are based on aprogrammed microprocessor or other programmed processing element, itwill be appreciated that not all of the steps of these techniques arenecessarily performed in a single microprocessor or even in a singlemodule. Embodiments of the presently disclosed techniques furtherinclude computer program products for application in a wireless terminalas well as corresponding computer program products for application in abase station apparatus or other network node apparatus.

This program code or computer program instructions may also be stored ina tangible computer-readable medium that can direct a computer or otherprogrammable data processing apparatus to function in a particularmanner, such that the instructions stored in the computer-readablemedium produce an article of manufacture including instructions whichimplement the functions/acts specified in the block diagrams and/orflowchart block or blocks. Accordingly, embodiments of present inventiveconcepts may be embodied in hardware and/or in software (includingfirmware, resident software, micro-code, etc.) running on a processorsuch as a digital signal processor, which may collectively be referredto as “circuitry,” “a module” or variants thereof.

It will further be appreciated that various aspects of theabove-described embodiments can be understood as being carried out byfunctional “modules,” which may be program instructions executing on anappropriate processor circuit, hard-coded digital circuitry and/oranalog circuitry, or appropriate combinations thereof.

For example, FIG. 15 illustrates an example functional module or circuitarchitecture as may be implemented in a UE 12, e.g., based on theprocessing circuit 30 and memory circuit 36. The illustrated embodimentat least functionally includes a signal generation module 1502 forgenerating a random access preamble signal. The implementation alsoincludes a transmission module 1504 for transmitting the random accesspreamble signal. The generated random access signal comprises two ormore consecutive preamble symbol groups, each preamble symbol groupcomprising a cyclic prefix portion and a plurality of identical symbolsoccupying a single subcarrier of the SC-FDMA random access preamblesignal, such that the single subcarrier for at least one of the preamblesymbol groups corresponds to a first subcarrier frequency and the singlesubcarrier for an immediately subsequent one of the preamble symbolgroups corresponds to a second subcarrier frequency. In someembodiments, every second one of the preamble symbol groups correspondsto the second subcarrier frequency and each of the remaining preamblesymbol groups corresponds to the first subcarrier frequency.

FIG. 16 illustrates an example functional module or circuit architectureas may be implemented in a network node, such as a base station 10,e.g., based on the processing circuit 40 and memory circuit 46. Theillustrated embodiment at least functionally includes a receiving module1602 for receiving a radio-frequency signal. The implementation alsoincludes a detection module 1604 for detecting, in the radio-frequencysignal, a first SC-FDMA random access preamble signal, transmitted by afirst remote radio device. The first random access preamble signalcomprises two or more consecutive preamble symbol groups, each preamblesymbol group comprising a cyclic prefix portion and a plurality ofidentical symbols occupying a single subcarrier of the SC-FDMA randomaccess preamble signal, such that the single subcarrier for at least oneof the preamble symbol groups corresponds to a first subcarrierfrequency and the single subcarrier for an immediately subsequent one ofthe preamble symbol groups corresponds to a second subcarrier frequency.In some embodiments, for example, every second one of the preamblesymbol groups corresponds to the second subcarrier frequency and each ofthe remaining preamble symbol groups corresponds to the first subcarrierfrequency.

Modifications and other variants of the described embodiment(s) willcome to mind to one skilled in the art having the benefit of theteachings presented in the foregoing descriptions and the associatedfigures. Therefore, it is to be understood that the embodiment(s) is/arenot to be limited to the specific examples disclosed and thatmodifications and other variants are intended to be included within thescope of this disclosure. Although specific terms may be employedherein, they are used in a generic and descriptive sense only and notfor purposes of limitation.

Example embodiments are listed below. It should be understood that theseare examples only; other embodiments and variants of the listedembodiments will be apparent from the detailed description providedabove. Embodiments of the techniques and apparatus described aboveinclude, but are not limited to, the following enumerated examples.

Example 1

A method, in a radio device, the method comprising: generating a randomaccess preamble signal; and transmitting the random access preamblesignal; wherein generating the random access preamble signal comprisesconcatenating N preamble symbols, each preamble symbol comprising asingle tone, wherein the single tone for every second preamble symbolcorresponds to a first subcarrier frequency and the single tone for theremaining preamble symbols corresponds to a second subcarrier frequency.

Example 2

The method of example 1, wherein all of the preamble symbols have thesame complex amplitude, wherein the first and second subcarrierfrequencies are selected so as to enable phase continuity at theboundaries between preamble symbols, and wherein the preamble symbolsare generated so as to provide phase continuity at the boundariesbetween preamble symbols.

Example 3

The method of example 1 or 2, wherein each preamble symbol has a cyclicprefix portion and a subsequent data portion, the cyclic prefix portionhaving a first length and being a duplicate of a concluding part of thesubsequent data portion.

Example 4

The method of example 3, wherein the subsequent data portion consists ofthree identical copies of the cyclic prefix portion.

Example 5

The method of example 4, wherein each preamble symbol has a total lengthof 1600 microseconds and the first and second subcarrier frequenciesdiffer by 2500 Hertz.

Example 6

The method of example 3, wherein the subsequent data portion has asecond length, the second length being four times the first time.

Example 7

The method of example 6, wherein each preamble symbol has a total lengthof 500 microseconds and wherein the first and second subcarrierfrequencies differ by 10 kHz.

Example 8

The method of any of examples 1-7, wherein N=100.

Example 9

The method of any of examples 1-8, the method further comprisingselecting a preamble configuration from a plurality of pre-determinedpreamble configurations, wherein the selected preamble configurationdefines the first and second subcarrier frequencies.

Example 10

A method, in a radio device, the method comprising: receiving aradio-frequency signal; and detecting, in the radio-frequency signal, afirst random access preamble signal, transmitted by a first remote radiodevice, wherein the first random access preamble signal comprises Nconcatenated preamble symbols, each preamble symbol comprising a singletone, wherein the single tone for every second one of the N preamblesymbols corresponds to a first subcarrier frequency and the single tonefor the remaining preamble symbols corresponds to a second subcarrierfrequency.

Example 11

The method of example 10, wherein all of the preamble symbols have thesame complex amplitude, wherein the first and second subcarrierfrequencies are selected so as to enable phase continuity at theboundaries between preamble symbols, and wherein the detected preamblesymbols have phase continuity at the boundaries between preamblesymbols.

Example 12

The method of example 10 or 11, wherein each preamble symbol has acyclic prefix portion and a subsequent data portion, the cyclic prefixportion having a first length and being a duplicate of a concluding partof the subsequent data portion.

Example 13

The method of example 12, wherein the subsequent data portion consistsof three identical copies of the cyclic prefix portion, and whereindetecting the first random access preamble signal comprises coherentlycombining three consecutive intervals in each preamble symbol.

Example 14

The method of example 13, wherein each preamble symbol has a totallength of 1600 microseconds and the first and second subcarrierfrequencies differ by 2500 Hertz.

Example 15

The method of example 12, wherein the subsequent data portion has asecond length, the second length being four times the first time.

Example 16

The method of example 15, wherein each preamble symbol has a totallength of 500 microseconds and wherein the first and second subcarrierfrequencies differ by 10 kHz.

Example 17

The method of any of examples 10-16, wherein N=100.

Example 18

The method of any of examples 10-17, the method further comprisingestimating a time-of-arrival for the first preamble signal.

Example 19

The method of any of examples 10-18, further comprising detecting, inthe radio-frequency signal, a second random access preamble signal,transmitted by a second remote radio device, wherein: the second randomaccess preamble signal comprises N concatenated preamble symbols, eachpreamble symbol of the second random access preamble signal comprising asingle tone; wherein the single tone for every second one of the Npreamble symbols of the second random access preamble corresponds to athird subcarrier frequency and the single tone for the remainingpreamble symbols of the second random access preamble corresponds to afourth subcarrier frequency; wherein the N preamble symbols of thesecond random access preamble overlap, at least partly, the N preamblesymbols of the first random access preamble; and wherein the firstsubcarrier frequency equals the fourth subcarrier frequency, or thesecond subcarrier frequency equals the third subcarrier frequency, orboth.

Example 20

A radio device comprising a radio transceiver adapted to communicatewith another radio device and further comprising one or more processingcircuits adapted to carry out the method of any of examples 1-9.

Example 21

A radio device comprising a radio transceiver adapted to communicatewith another radio device and further comprising one or more processingcircuits adapted to carry out the method of any of examples 10-19.

Example 22

A radio device adapted to: generate a random access preamble signal byconcatenating N preamble symbols, each preamble symbol comprising asingle tone, wherein the single tone for every second preamble symbolcorresponds to a first subcarrier frequency and the single tone for theremaining preamble symbols corresponds to a second subcarrier frequency;and transmit the random access preamble signal.

Example 23

A radio device comprising: a signal generation module for generating arandom access preamble signal by concatenating N preamble symbols, eachpreamble symbol comprising a single tone, wherein the single tone forevery second preamble symbol corresponds to a first subcarrier frequencyand the single tone for the remaining preamble symbols corresponds to asecond subcarrier frequency module; and a transmission module fortransmitting the random access preamble signal.

Example 24

A radio device adapted to: receive a radio-frequency signal; and detect,in the radio-frequency signal, a first random access preamble signal,transmitted by a first remote radio device, wherein the first randomaccess preamble signal comprises N concatenated preamble symbols, eachpreamble symbol comprising a single tone, wherein the single tone forevery second one of the N preamble symbols corresponds to a firstsubcarrier frequency and the single tone for the remaining preamblesymbols corresponds to a second subcarrier frequency.

Example 25

A radio device, comprising: a receiving module for receiving aradio-frequency signal; and a detection module for detecting, in theradio-frequency signal, a first random access preamble signal,transmitted by a first remote radio device, wherein the first randomaccess preamble signal comprises N concatenated preamble symbols, eachpreamble symbol comprising a single tone, wherein the single tone forevery second one of the N preamble symbols corresponds to a firstsubcarrier frequency and the single tone for the remaining preamblesymbols corresponds to a second subcarrier frequency.

Example 26

A computer program product comprising program instructions for aprocessor in a radio device, wherein said program instructions areconfigured so as to cause the radio device when the program instructionsare executed by the processor, to: generate a random access preamblesignal by concatenating N preamble symbols, each preamble symbolcomprising a single tone, wherein the single tone for every secondpreamble symbol corresponds to a first subcarrier frequency and thesingle tone for the remaining preamble symbols corresponds to a secondsubcarrier frequency; and transmit the random access preamble signal.

Example 27

A non-transitory computer-readable medium comprising, stored thereupon,the computer program product of example 26.

Example 28

A computer program product comprising program instructions for aprocessor in a radio device, wherein said program instructions areconfigured so as to cause the radio device when the program instructionsare executed by the processor, to: receive a radio-frequency signal; anddetect, in the radio-frequency signal, a first random access preamblesignal, transmitted by a first remote radio device, wherein the firstrandom access preamble signal comprises N concatenated preamble symbols,each preamble symbol comprising a single tone, wherein the single tonefor every second one of the N preamble symbols corresponds to a firstsubcarrier frequency and the single tone for the remaining preamblesymbols corresponds to a second subcarrier frequency.

Example 29

A non-transitory computer-readable medium comprising, stored thereupon,the computer program product of example 28.

A draft of a contribution to standardization efforts in 3GPP has beendeveloped, and was included as a part of the provisional patentapplication to which the present application claim claims priority. Theentirety of that draft contribution, labeled “Appendix A” in theprovisional application, is incorporated by reference herein, for thepurpose of providing non-limiting examples of how the inventivetechniques described herein may be applied, and in particular to provideillustrative examples of the nature of changes that might be made towireless communications standards, based on these inventive techniques.

Several methods, devices, and systems for generating and receivingrandom access preambles have been described in detail above. It will beappreciated by persons of ordinary skill in the art that the embodimentsencompassed by the present disclosure are not limited to the particularexemplary embodiments described above. In that regard, althoughillustrative embodiments have been shown and described, a wide range ofmodification, change, and substitution is contemplated in the foregoingdisclosure. It is understood that such variations may be made to theforegoing without departing from the scope of the present disclosure.Accordingly, it is appropriate that the appended claims be construedbroadly and in a manner consistent with the present disclosure.

What is claimed is:
 1. A method, in a user equipment, the methodcomprising: generating a random access preamble signal comprising two ormore consecutive preamble symbol groups, each preamble symbol groupcomprising a plurality of identical symbols occupying a singlesubcarrier of the random access preamble signal, such that the singlesubcarrier for at least one of the preamble symbol groups corresponds toa first subcarrier frequency and the single subcarrier for animmediately subsequent one of the preamble symbol groups corresponds toa second subcarrier frequency; and transmitting the random accesspreamble signal.
 2. The method of claim 1, wherein the second subcarrierfrequency is adjacent to the first subcarrier frequency.
 3. The methodof claim 1, wherein phase continuity exists at the boundary between theat least one of the preamble symbol groups and the immediatelysubsequent one of the preamble symbol groups.
 4. A method, in a basestation, the method comprising: receiving a radio-frequency signal; anddetecting, in the radio-frequency signal, a first random access preamblesignal, transmitted by a first remote user equipment, wherein the firstrandom access preamble signal comprises two or more consecutive preamblesymbol groups, each preamble symbol group comprising a plurality ofidentical symbols occupying a single subcarrier of the first randomaccess preamble signal, such that the single subcarrier for at least oneof the preamble symbol groups corresponds to a first subcarrierfrequency and the single subcarrier for an immediately subsequent one ofthe preamble symbol groups corresponds to a second subcarrier frequency.5. The method of claim 4, wherein the second subcarrier frequency isadjacent to the first subcarrier frequency.
 6. The method of claim 4,wherein phase continuity exists at the boundary between the at least oneof the preamble symbol groups and the immediately subsequent one of thepreamble symbol groups.
 7. A user equipment comprising: one or moreprocessing circuits configured to generate a random access preamblesignal comprising two or more consecutive preamble symbol groups, eachpreamble symbol group comprising a plurality of identical symbolsoccupying a single subcarrier of the random access preamble signal, suchthat the single subcarrier for at least one of the preamble symbolgroups corresponds to a first subcarrier frequency and the singlesubcarrier for an immediately subsequent one of the preamble symbolgroups corresponds to a second subcarrier frequency, and transmit therandom access preamble signal from the user equipment to a base station.8. The user equipment of claim 7, further comprising: a radiotransceiver adapted to communicate with the base station, and whereinthe one or more processing circuits are further configured to transmitthe random access preamble signal from the user equipment to the basestation via the radio transceiver.
 9. The user equipment of claim 7,wherein the second subcarrier frequency is adjacent to the firstsubcarrier frequency.
 10. The user equipment of claim 7, wherein the oneor more processing circuits are configured to generate the random accesspreamble signal such that phase continuity exists at the boundarybetween the at least one of the preamble symbol groups and theimmediately subsequent one of the preamble symbol groups.
 11. The userequipment of claim 7, wherein the one or more processing circuits areconfigured to generate the random access preamble signal such that allof the preamble symbol groups in the random access preamble signal havethe same complex amplitude.
 12. The user equipment of claim 7, whereinthe one or more processing circuits are further configured to generatingthe random access preamble signal using a preamble configurationselected from a plurality of pre-determined preamble configurations,wherein the selected preamble configuration defines at least the firstand second subcarrier frequencies.
 13. A base station comprising: aradio transceiver configured to communicate with one or more remote userequipments; and one or more processing circuits configured to: receive aradio-frequency signal, using the radio transceiver; and detect, in theradio-frequency signal, a first random access preamble signal,transmitted by a first remote user equipment, wherein the first randomaccess preamble signal comprises two or more consecutive preamble symbolgroups, each preamble symbol group comprising a plurality of identicalsymbols occupying a single subcarrier of the first random accesspreamble signal, such that the single subcarrier for at least one of thepreamble symbol groups corresponds to a first subcarrier frequency andthe single subcarrier for an immediately subsequent one of the preamblesymbol groups corresponds to a second subcarrier frequency.
 14. The basestation of claim 13, wherein the second subcarrier frequency is adjacentto the first subcarrier frequency.
 15. The base station of claim 13,wherein phase continuity exists at the boundary between the at least oneof the preamble symbol groups and the immediately subsequent one of thepreamble symbol groups.
 16. The base station of claim 13, wherein all ofthe preamble symbol groups in the first random access preamble signalhave the same complex amplitude.
 17. The base station of claim 13,wherein each preamble symbol group has a total length of 1600microseconds.
 18. The base station of claim 13, wherein the one or moreprocessing circuits are further configured to estimate a time-of-arrivalfor the first preamble signal.
 19. The base station of claim 13, whereinthe one or more processing circuits are further configured to detect, inthe radio-frequency signal, a second random access preamble signal,transmitted by a second remote user equipment, wherein: the secondrandom access preamble signal comprises two or more consecutive preamblesymbol groups, each preamble symbol group comprising a plurality ofidentical symbols occupying a single subcarrier of the second randomaccess preamble signal, such that the single subcarrier for at least oneof the preamble symbol groups corresponds to a third subcarrierfrequency and the single subcarrier for an immediately subsequent one ofthe preamble symbol groups corresponds to a fourth subcarrier frequency;wherein the two or more consecutive preamble symbol groups of the secondrandom access preamble signal overlap, at least partly, the two or moreconsecutive preamble symbol groups of the first random access preamble;and wherein the first subcarrier frequency equals the fourth subcarrierfrequency, or the second subcarrier frequency equals the thirdsubcarrier frequency, or both.